1.3 membrane structure

Question 1

what is the Phospholipid bilayer?

Answer 1

• separate the internal contents of the cell from the outside of the cell
• cell membrane is made of phospholipids.
• Phospholipids are amphipathic as it has both hydrophilic (heads) and hydrophobic (tails) properties

Question 2

different models of the bilayer.

Answer 2

• Gorter & Grendel concluded that
  cell membranes were phospholipid
  bilayers, but their model lacked the
  presence of proteins.
• Davson-Danielli model had the phospholipid bilayer in between a layer of proteins

Question 3

how was the Davson-Danielli model falsified?

Answer 3

• Freeze-etched electron micrographs showed fractures in the centres of the membranes that was interpreted as transmembrane (integral) proteins.
• Membrane proteins were extracted and shown to vary in size and globular shape. Membrane proteins also had hydrophilic and hydrophobic regions that may be attracted to different parts of phospholipids
• Fluorescent antibody tagging of membrane proteins of different cells (red or green), and following cell fusion, the fluorescent colour was thoroughly mixed suggesting that membrane proteins are free to move through the membrane

Question 4

what is the fluid mosaic model?

Answer 4

• proposed by singer and nicolson
• biological membranes were composed of a phospholipid
  bilayer with proteins embedded either halfway through or all the way through the membrane

Question 5

how to draw the bilayer?

Answer 5

• The individual phospholipid molecules should be drawn using a circle and two parallel lines.
• A range of membrane proteins should be shown including glycoproteins, integral proteins and peripheral proteins.
• Cholesterol should be included in animal cell membrane diagrams.

Question 6

what are integral proteins?

Answer 6

• (transmembrane) are embedded in and span across the bilayer of the membrane with regions of hydrophilic parts projecting out to either side of the membrane

Question 7

what are Peripheral proteins?

Answer 7

• have a hydrophilic surface and are not embedded in the membrane but rather attached to integral proteins

Question 8

what are the six main functions of membrane proteins?

Answer 8

• Hormone binding sites: Glycoproteins that are exposed on the membrane surface allow one specific hormone to bind. The binding transmits a signal inside the cell e.g. insulin receptor.
• Immobilised enzymes: Enzymes have an active side on the outside of the membrane to speed up biochemical reactions e.g. in the small intestine.
• Cell adhesion: Integral proteins bind to specific proteins in adjacent tissues and organs to form tight junctions.
• Cell-to-cell communication: Can be via direct contact or via a hormone (insulin) or neurotransmitter (adrenaline).
• Channels for passive transport: Facilitates movement of hydrophilic substances across the membrane.
• Pumps for active transport: Facilitates movement of substances across the membrane with the use of ATP (energy).

Question 9

what does Cholesterol in animal cell membranes do?

Answer 9

• Animal cell membranes contain cholesterol
• Cholesterol is a type of lipid that belongs to substances called steroids.
• Cholesterol is an amphipathic molecule
• The hydrophobic region is attracted to the hydrocarbon tails of the phospholipid.
• The hydroxyl (-OH) group is hydrophilic and is attracted to the phosphate heads of phospholipids

Question 10

what does cholesterol do in the bilayer?

Answer 10

• prevents the membrane from becoming too solid, which allows materials to pass through the phospholipids whilst preventing the membrane from becoming too fluid
  by maintaining the shape
• It reduces the permeability of sodium and hydrogen ions.
• It also helps in the formation of vesicles during endocytosis